1. Field of the Invention
The present invention relates to a room temperature-curable, heat-conductive silicone rubber composition that is able to suppress the quantity of hydrogen gas produced from the heat-conductive filler, and exhibits superior storage properties.
2. Description of the Prior Art
Heat-generating components such as power transistors and thyristors suffer from a deterioration in their properties as a result of heat generation, and conventionally, when these types of components are installed, countermeasures such as attaching a heat sink to the component to disperse the heat, or allowing the heat to escape into the metal chassis of the instrument have been employed. In such cases, in order to improve the electrical insulation properties and the thermal conductivity, a heat-dissipating, insulating sheet comprising a silicone rubber containing a heat-conductive filler is generally used between the heat-generating component and the heat sink.
As an example of the heat-dissipating, insulating material, patent reference 1 discloses an insulating composition comprising 100 parts by mass of a synthetic rubber such as a silicone rubber blended with 100 to 800 parts by mass of at least one metal oxide selected from the group consisting of beryllium oxide, aluminum oxide, hydrated aluminum oxide, magnesium oxide, and zinc oxide.
Furthermore, as an example of a heat-dissipating material that can be used in locations that do not require insulation, patent reference 2 discloses a composition comprising an addition-curable silicone rubber composition blended with 60 to 500 parts by mass of a silica and a heat-conductive powder such as copper, silver, gold or silicon.
However, the level of integration within electrical devices such as personal computers, word processors and CD-ROM drives continues to increase, and the quantity of heat generated by the integrated circuit elements such as LSI and CPU units within these devices has also increased, meaning conventional cooling methods are sometimes no longer adequate. Particularly in the case of portable notebook computers, because the space inside the device is restricted, large heat sinks or cooling fans cannot be fitted. In these types of devices, the integrated circuit elements are mounted on a printed substrate, and because a material with poor thermal conductivity such as a glass-reinforced epoxy resin or polyimide resin or the like is used as the substrate material, the conventional method of allowing the heat to escape into the substrate via a heat-dissipating, insulating sheet cannot be used.
Accordingly, a liquid-type silicone rubber composition is used. An addition-curable (heat-curable) silicone rubber composition requires the use of a heating device during curing, but the limited heat resistance of the integrated circuit elements means that heating cannot be conducted at high temperatures of 60° C. or higher, and the introduction of a heating device also requires additional investment in equipment. Furthermore, because the produced device is used at room temperature following heat curing, strain (stress) is exerted on the integrated circuit elements.
Furthermore, even in those cases where a condensation-curable (room temperature-curable) silicone rubber composition is used, if a large quantity of a heat-conductive filler is blended into the composition to improve the thermal conductivity, then the hardness level following curing of the silicone rubber composition tends to be overly high, which can also exert strain (stress) on the integrated circuit elements.
In order to increase the thermal conductivity of these types of heat-conductive silicone rubber compositions without increasing the hardness, copper powder or silver powder may be used as the heat-conductive filler, but because these heat-conductive fillers are extremely expensive, the resulting silicone rubber composition product is also very expensive. As a result, large numbers of silicone rubber compositions that use comparatively cheap aluminum powder have been developed. However, because of its reactivity, aluminum powder tends to generate hydrogen gas in the presence of acidic components, alkaline components, or moisture or the like.
If hydrogen gas is generated from a heat-conductive silicone rubber composition, then large quantities of gas bubbles derived from the hydrogen gas are formed within the composition, which can cause swelling or breakage of the container used for storing the heat-conductive silicone rubber composition.
On the other hand, palladium is widely known to have a property of adsorbing hydrogen gas.
[Patent Reference 1] JP 47-32400 A
[Patent Reference 2] EP 0 032 050 A2